Hopefully this real world example might help...
Our office LAN on IP range 192.168.0.x, including a managed VPN router supplied by ISP.
Home user has a VPN Router with a local IP range of 192.168.35.x, DHCP enabled. I used a Linksys RV042.
In IPO Manager, added an IP Route for IP Address 192.168.35.0 with gateway set to local IP of office VPN router.
On 5610 phone, set options to use DHCP, but with TFTP server and CallSv set to the IPO's LAN IP address (192.168.0.?)
Before I went to the home user's... um... home (150 miles away), I tried it at my house first since I had a similar setup on same ISP to the user. Took me a couple of hours to get it working, although that was the Linksys not connecting to office VPN... when VPN worked, phone connected automatically since I'd already programmed the IP settings.
Oh, and make sure you have a PSU for the phone unless you get a VPN router with power over ethernet
